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1.
Pyrite and marcasite were precipitated by both slow addition of aqueous Fe 2+ and SiO 32− to an H 2S solution and by mixing aqueous Fe 2+ and Na 2S 4 solutions at 75°C. H 2S 2 or HS 2− and H 2S 4 or HS 4− were formed in the S 2O 32− and Na 2S 4 experiments, respectively. Marcasite formed at pH < pK 1 of the polysulfide species present (for H2S2, pK1 = 5.0; for H2S4, pK1 = 3.8 at 25° C). Marcasite forms when the neutral sulfane is the dominant polysulfide, whereas pyrite forms when mono-or divalent polysulfides are dominant. In natural solutions where H 2S 2 and HS 2 are likely to be the dominant polysulfides, marcasite will form only below pH 5 at all temperatures. The pH-dependent precipitation of pyrite and marcasite may be caused by electrostatic interactions between polysulfide species and pyrite or marcasite growth surfaces: the protonated ends of H2S2 and HS2 are repelled from pyrite growth sites but not from marcasite growth sites. The negative ions HS2 and S22− are strongly attracted to the positive pyrite growth sites. Masking of 1πg* electrons in the S2 group by the protons makes HS2 and H2S2 isoelectronic with AsS2− and As22−, respectively (
et al., 1981). Thus, the loellingitederivative structure (marcasite) results when both ends of the polysulfide are protonated. Marcasite occurs abundantly only for conditions below pH 5 and where H2S2 was formed near the site of deposition by either partial oxidation of aqueous H2S by O2 or by the reaction of higher oxidation state sulfur species that are reactive with H2S at the conditions of formation e.g., S2O32− but not SO42−. The temperature of formation of natural marcasite may be as high as 240°C (
and
, 1985), but preservation on a multimillion-year scale seems to require post-depositional temperatures of below about 160°C (
, 1973;
and
, 1985). 相似文献
2.
We present results of computations on the interaction of solid-phase electrum–argentite–pyrite (weight ratios 210 −5/ 210 −3/1 and 210 −5/410 −2/1) association with Cl-containing aqueous moderately acid solutions (0.5m NaCl, pH = 3.08) at 300 °C and 500 bars. These data are a physicochemical basis for predicting the geochemical behavior of Au and Ag during the hydrothermal-metasomatic transformation of Au-Ag-pyrite. We also propose a technique of study of this process based on the phase equilibria of the subsystem Au–Ag–S with the aqueous solution at different liquid/solid (l/s) ratios, with the use of new graphic diagrams. The relationship of the composition of the solid-phase association with l/s ratio in real boundary conditions (Au = 17 ppm, mAu/mAg = 10 –3.57–10 –2.28) is shown. The maximum l/s values for complete leaching of gold and silver (l/smax = 200–800) are estimated. It has been established that argentite is the first to dissolve when mAu/mAg(s) > mAu/mAg(sol), and electrum, when mAu/mAg(s) < mAu/mAg(sol). The experimental results showed that at 300 °C, the conversion of electrum (NAu = 300‰) nonequilibrated with pyrite into an Au-richer form (NAu = 730‰) and argentite follows an intricate kinetic scheme. Using the Pilling-Bedwords kinetic equation for processing data yielded the process rate constant K = 2.8(±0.5)10−5 g2cm−4day−1. With this equation, the time of the complete conversion of 200 μm thick flat gold grains is 604 days. These data evidence a significant role of kinetic factors in hydrothermal-metasomatic processes involving native gold, which requires combination of thermodynamic and kinetic approaches on the construction of geologo-genetic models for hydrothermal sulfide formation. 相似文献
3.
The dissolution and precipitation rates of boehmite, AlOOH, at 100.3 °C and limited precipitation kinetics of gibbsite, Al(OH) 3, at 50.0 °C were measured in neutral to basic solutions at 0.1 molal ionic strength (NaCl + NaOH + NaAl(OH) 4) near-equilibrium using a pH-jump technique with a hydrogen-electrode concentration cell. This approach allowed relatively rapid reactions to be studied from under- and over-saturation by continuous in situ pH monitoring after addition of basic or acidic titrant, respectively, to a pre-equilibrated, well-stirred suspension of the solid powder. The magnitude of each perturbation was kept small to maintain near-equilibrium conditions. For the case of boehmite, multiple pH-jumps at different starting pHs from over- and under-saturated solutions gave the same observed, first order rate constant consistent with the simple or elementary reaction: . This relaxation technique allowed us to apply a steady-state approximation to the change in aluminum concentration within the overall principle of detailed balancing and gave a resulting mean rate constant, (2.2 ± 0.3) × 10−5 kg m−2 s−1, corresponding to a 1σ uncertainty of 15%, in good agreement with those obtained from the traditional approach of considering the rate of reaction as a function of saturation index. Using the more traditional treatment, all dissolution and precipitation data for boehmite at 100.3 °C were found to follow closely the simple rate expression: Rnet,boehmite=10-5.485{mOH-}{1-exp(ΔGr/RT)}, with Rnet in units of mol m−2 s−1. This is consistent with Transition State Theory for a reversible elementary reaction that is first order in OH− concentration involving a single critical activated complex. The relationship applies over the experimental ΔGr range of 0.4–5.5 kJ mol−1 for precipitation and −0.1 to −1.9 kJ mol−1 for dissolution, and the pHm ≡ −log(mH+) range of 6–9.6. The gibbsite precipitation data at 50 °C could also be treated adequately with the same model:Rnet,gibbsite=10-5.86{mOH-}{1-exp(ΔGr/RT)}, over a more limited experimental range of ΔGr (0.7–3.7 kJ mol−1) and pHm (8.2–9.7). 相似文献
4.
Within the framework of Pitzer's specific interaction model, interaction parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solubility measurements. The values, at 25°C, of the Pitzer interaction parameter (λ SiO2(aq)−i) determined in this study are the following: 0.092 (i = Na +), 0.032 (K +), 0.165 (Li +), 0.292 (Ca 2+, Mg 2+), −0.139 (SO 42−), and −0.009 (NO 3−). A set of polynomial equations has been derived which can be used to calculate λ SiO2(aq)−i for these ions at any temperature up to 250°C. A linear relationship between the aqueous silica-ion interaction parameters (λ SiO2(aq)−i) and the surface electrostatic field ( Zi/ re,i) of ions was obtained. This empirical equation can be used to estimate, in first approximation, λ SiO2(aq)−i if no measurements are available. From this parameterisation, the calculated activity coefficient of aqueous silica is 2.52 at 25°C and 1.45 at 250°C in 5 m NaCl solution. At lower concentrations, e.g. 2 m NaCl, the activity coefficient of silica is 1.45 at 25°C and 1.2 at 250°C. Hence, in practice, it is necessary to take into account the activity coefficient of aqueous silica (λ SiO2(aq)≠1) in hydrothermal solutions and basinal brines where the ionic strength exceeds 1. A comparison of measured [Marshall, W.L., Chen, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solubility behaviour in aqueous mixed electrolyte solutions to 300°C. Geochim. Cosmochim. Acta 46, 289–291.] and computed amorphous silica solubility, using this parameterisation, shows a good agreement. Because the effect of individual ions on silicate and silica polymorph solubilities are additive, the present study has permitted to derive Pitzer interaction parameters that allow a precise computation of γ SiO2(aq) in the Na---K---Ca---Mg---Cl---SO 4---HCO 3---SiO 2---H 2O system, over a large range of salt concentrations and up to temperatures of 250°C. 相似文献
5.
The δD SMOW values of sedimentary kaolins from the western border of the Bohemian Massif, northeast Bavaria, that did not suffer a deep burial (less than 1000 m) nor a hydrothermal overprint, change systematically from Late Triassic (−50‰) to Mid-Jurassic and Late Cretaceous (−56‰ to −66‰) to Upper Oligocene–Mid-Miocene (−77‰ to −90‰). All analyzed clays are far from hydrogen isotope equilibrium with present-day meteoric waters. Combined oxygen and hydrogen isotope data of selected samples indicate low temperatures of formation (<30°C) and no evidence for preferential D/H exchange with younger waters. The hydrogen isotopic evolution of kaolins is interpreted as reflecting a systematic isotopic change of paleo-meteoric waters in that region. This can be related mainly to the northward drift of stable Europe after the break-up of Gondwana. Increasing continentality, surface uplift and global cooling are additional factors responsible for decreasing δD SMOW values since the Mid-Cretaceous. Kaolinite hydrogen isotope ratios of two large residual economic deposits (Tirschenreuth: δDSMOW=−80‰ to −76‰; Hirschau–Schaittenbach: δDSMOW=−70‰ to −63‰) can be used in combination with additional geological evidence to constrain the timing of weathering in these areas. A late Early Cretaceous kaolinization age is suggested for the Early Triassic sandstone-hosted deposits near Hirschau–Schnaittenbach, whereas a Late Oligocene to Mid-Miocene age is indicated for the Carboniferous granite-hosted Tirschenreuth deposits. 相似文献
6.
Dissolution rates of single calcite crystals were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed at 25 °C in aqueous HCl solutions over the bulk solution pH range −1 to 3 and in the presence of trace concentrations of aqueous NaPO 3 and MgCl 2. These salts were chosen for this study because aqueous magnesium and phosphate are known to strongly inhibit calcite dissolution at neutral to basic pH. Reactive solutions were undersaturated with respect to possible secondary phases. Neither an inhibition or enhancement of calcite dissolution rates was observed in the presence of aqueous MgCl 2 at pH 1 and 3. The presence of trace quantities of NaPO 3, which dissociates in solution to Na + and H 2PO 4−, decreased the overall calcite dissolution rate at pH≤2. This contrasting behavior could be attributed to the different adsorption behavior of these dissolved species. As calcite surfaces are positively charged in acidic solutions, aqueous Mg 2+ may not adsorb, whereas aqueous phosphate, present as either the anion H 2PO 4− or the neutral species H 3PO 40, readily adsorbs on calcite surfaces leading to significant dissolution inhibition. 相似文献
7.
A detailed fluid inclusion study has been carried out on the hydrocarbon-bearing fluids found in the peralkaline complex, Lovozero. Petrographic, microthermometric, laser Raman and bulk gas data are presented and discussed in context with previously published data from Lovozero and similar hydrocarbon-bearing alkaline complexes in order to further understand the processes which have generated these hydrocarbons. CH 4-dominated inclusions have been identified in all Lovozero samples. They occur predominantly as secondary inclusions trapped along cleavage planes and healed fractures together with rare H 2O-dominant inclusions. They are consistently observed in close association with either arfvedsonite crystals, partially replaced by aegirine, aegirine crystals or areas of zeolitization. The majority of inclusions consist of a low-density fluid with CH 4 homogenisation temperatures between −25 and −120 °C. Those in near-surface hand specimens contain CH 4+H 2 (up to 40 mol%)±higher hydrocarbons. However, inclusions in borehole samples contain CH 4+higher hydrocarbons±H 2 indicating that, at depth, higher hydrocarbons are more likely to form. Estimated entrapment temperatures and pressures for these inclusions are 350 °C and 0.2–0.7 kbar. A population of high-density, liquid, CH 4-dominant inclusions have also been recorded, mainly in the borehole samples, homogenising between −78 and −99 °C. These consist of pure CH 4, trapped between 1.2 and 2.1 kbar and may represent an early CH 4-bearing fluid overprinted by the low-density population. The microthermometric and laser Raman data are in agreement with bulk gas data, which have recorded significant concentrations of H 2 and higher hydrocarbons up to C 6H 12 in these samples. These data, combined with published isotopic data for the gases CH 4, C 2H 6, H 2, He and Ar indicate that these hydrocarbons have an abiogenic, crustal origin and were generated during postmagmatic, low temperature, alteration reactions of the mineral assemblage. This would suggest that these data favour a model for formation of hydrocarbons through Fischer–Tropsch type reactions involving an early CO 2-rich fluid and H 2 derived from alteration reactions. This is in contrast to the late-magmatic model suggested for the formation of hydrocarbons in the similar peralkaline intrusion, Ilímaussaq, at temperatures between 400 and 500 °C. 相似文献
8.
As a result of the collapse of a mine tailing dam, a large extension of the Guadiamar valley was covered with a layer of pyritic sludge. Despite the removal of most of the sludge, a small amount remained in the soil, constituting a potential risk of water contamination. The kinetics of the sludge oxidation was studied by means of laboratory flow-through experiments at different pH and oxygen pressures. The sludge is composed mainly of pyrite (76%), together with quartz, gypsum, clays, and sulphides of zinc, copper, and lead. Trace elements, such as arsenic and cadmium, also constitute a potential source of pollution. The sludge is fine grained (median of 12 μm) and exhibits a large surface (BET area of 1.4±0.2 m 2 g −1). The dissolution rate law of sludge obtained is r=10−6.1(±0.3) [O2(aq)]0.41(±0.04) aH+0.09(±0.06) gsludge m−2 s−1 (22 °C, pH=2.5–4.7). The dissolution rate law of pyrite obtained is r=10−7.8(±0.3) [O2(aq)]0.50(±0.04) aH+0.10(±0.08) mol m−2 s−1 (22 °C, pH=2.5–4.7). Under the same experimental conditions, sphalerite dissolved faster than pyrite but chalcopyrite dissolves at a rate similar to that of pyrite. No clear dependence on pH or oxygen pressure was observed. Only galena dissolution seemed to be promoted by proton activity. Arsenic and antimony were released consistently with sulphate, except at low pH conditions under which they were released faster, suggesting that additional sources other than pyrite such as arsenopyrite could be present in the sludge. Cobalt dissolved congruently with pyrite, but Tl and Cd seemed to be related to galena and sphalerite, respectively. A mechanism for pyrite dissolution where the rate-limiting step is the surface oxidation of sulphide to sulphate after the adsorption of O2 onto pyrite surface is proposed. 相似文献
9.
The gas and redox chemistry of 100–300 °C geothermal fluids in Iceland has been studied as a function of fluid temperature and fluid composition. The partial pressures of CO 2 in dilute ( mCl<500 ppm) and saline ( mCl>500 ppm) geothermal fluids above 200 °C are controlled by the mineral buffer clinozoisite+prehnite+calcite+quartz. Two buffers are considered to control the H 2S and H 2 partial pressures above 200 °C depending on fluid salinity, epidote+prehnite+pyrite+pyrrhotite for dilute fluids and pyrite+prehnite+quartz+magnetite+anhydrite+clinozoisite+quartz for saline fluids. Below 200 °C, the partial pressures of CO 2, H 2S and H 2 also seem to be buffered but other minerals must be involved. Zeolites are expected to replace prehnite and epidote. Redox potential calculated on the assumption of equilibrium for the H +/H 2 redox couple decreases in dilute geothermal fluids with increasing temperature from about −0.5 V at 100 °C to −0.8 V at 300 °C, whereas saline geothermal fluids at 250 °C display a redox potential of about −0.45 V. A systematic discrepancy between redox couples of about 0.05–0.09 V is observed in the redox potential for the dilute geothermal fluids, whereas redox potentials agree within 0.02–0.04 V for saline geothermal waters. The discrepancies in the calculated redox potential for dilute geothermal fluids are thought to be due to a general lack of equilibrium between CH 4, CO 2 and H 2 and between H 2S, SO 4 and H 2. It is, accordingly, concluded that an overall equilibrium among redox species has not been reached for dilute geothermal fluids whereas it appears to be more closely approached for the saline geothermal fluids. The latter conclusion is based on limited database and should be treated with care. Since the various redox components are not in an overall equilibrium in geothermal fluids in Iceland these fluids cannot be characterised by a unique hydrogen fugacity, oxygen fugacity or redox potential at a given temperature and pressure. 相似文献
10.
Experimental research on K-rich phases and observations from diamond inclusions, UHP metamorphic rocks, and xenoliths provide insights about the hosts for potassium at mantle conditions. K-rich clinopyroxene (Kcpx–KM 3+Si 2O 6) can be an important component in clinopyroxenes at P>4 GPa, dependent upon coexisting K-bearing phases (solid or liquid) but not, apparently, upon temperature. Maximum Kcpx content can reach 25 mol%, with 17 mol% the highest reported in nature. Partitioning (K)D(cpx/liquid) above 7 GPa=0.1–0.2 require ultrapotassic liquids to form highly potassic cpx or critical solid reactions, e.g., between Kspar and Di. Phlogopite can be stable to about 8 GPa at 1250 °C where either amphibole or liquid forms. When fluorine is present, it generally increases in Phl upon increasing P (and probably T) to about 6 GPa, but reactions forming amphibole and/or KMgF 3 limit F content between 6 and 8 GPa. The perovskite KMgF 3 is stable up to 10 GPa and 1400 °C as subsolidus breakdown products of phlogopite upon increasing P. (M4)K-substituted potassic richterite (ideally K(KCa)Mg 5Si 8O 22(OH,F) 2) is produced in K-rich peridotites above 6 GPa and in Di+Phl from 6 to 13 GPa. K content of amphibole is positively correlated with P; Al and F content decrease with P. In the system 1Kspar+1H 2O K-cymrite (hydrous hexasanidine–KAlSi 3O 8· nH 2O–Kcym) is stable from 2.5 GPa at 400 to 1200 °C and 9 GPa; Kcym can be a supersolidus phase. Formation of Kcym is sensitive to water content, not forming within experiments with H 2O 2O>Kspar. Phase X, a potassium di-magnesium acid disilicate ((K1−x−n)2(Mg1−nMn3+)2Si2O7H2x), forms in mafic compositions at T=1150–1400 °C and P=9–17 GPa and is a potential host for K and H2O at mantle conditions with a low-T geotherm or in subducting slabs. The composition of phase-X is not fixed but actually represents a solid solution in the stoichiometries □2Mg2Si2O7H2–(K□)Mg2Si2O7H–K2Mg2Si2O7 (□=vacancy), apparently stable only near the central composition. K-hollandite, KAlSi3O8, is possibly the most important K-rich phase at very high pressure, as it appears to be stable to conditions near the core–mantle boundary, 95 GPa and 2300 °C. Other K-rich phases are considered. 相似文献
11.
Cordierite samples from pegmatites and metamorphic rocks have been analysed for major [electron microprobe analysis (EMPA)] and trace elements [inductively coupled plasma mass spectrometry (ICP-MS), secondary ion mass spectrometry analyses (SIMS)] as well as for H 2O and CO 2 (coulometric titration), and the results evaluated in conjunction with published data in order to determine which exchange mechanisms are significant. Apart from the homovalent substitutions FeMg −1 and MnMg −1 on the octahedral site, some minor KNa −1 on the Ch0 channel site, and Fe 3+Al −1 on the T 11 tetrahedral site, the three most important substitution mechanisms are those for the incorporation of Li on the octahedral sites (NaLi□ −1Mg −1), and of Be and other divalent cations on the tetrahedral T 11 site (NaBe□ −1Al −1 and Na(Mg,Fe 2+)□ −1Al −1). The dominant role of the last vector is clearly demonstrated. We propose a new generalized formula for cordierite: Ch(Na,K) 0–1 VI(Mg,Fe 2+,Mn,Li) 2 IVSi 5 IVAl 3 IV(Al, Be, Mg, Fe 2+, Fe 3+)O 18 * xCh(H 2O, CO 2…). Our results show that the population of (Mg, Fe 2+) on the T 11-site is limited to about 0.08 a.p.f.u. Other exchange mechanisms that were encountered in experiments operate only under P– T conditions or in bulk compositions that are rarely realized in nature. Routine analyses by electron microprobe in which Li and Be are not determined can be plotted as (Mg+Fe+Mn) versus (Si+Al) to assess whether significant amounts of Li and Be could be present. These amounts can be calculated as Li (a.p.f.u.)=Al+Na–4 and Be (a.p.f.u.)=10–2Al–M 2+–Na. 相似文献
12.
As is typical of ultra-high pressure (UHP) terrains, the regional extent of the UHP terrain in the Dabieshan of central China is highly speculative, since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and/or diamond stability is very small. By contrast, the common garnet ( XMn=0.18–0.45)–phengite (Si=3.2–3.35)–zoned epidote (Ps 38–97)–biotite–titanite–two feldspars–quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low P– T conditions of ca. 400±50°C at 4 kbar. However, certain orthogneiss samples preserve garnets with XCa up to 0.50, rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with Si contents p.f.u. of up to 3.49 — overlapping with the highest values (3.49–3.62) recorded for phengites in samples of undoubted UHP schists. These and other mineral composition features (such as A-site deficiencies in the highest Si phengites, Na in garnets linked to Y+Yb substitution and Al F Ti −1 O −1 substitution in titanites) are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated UHP schists and eclogites. Re-evaluated garnet–phengite and garnet–biotite Fe/Mg exchange thermometry and calculated 5 rutile+3 grossular+2SiO 2+H 2O=5 titanite+2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction-related clockwise P– T path with the UHP paragneisses and eclogites through conditions of Pmax at ca. 690°C–715°C and 36 kbar to Tmax at ca. 710°C–755°C and 18 kbar, prior to extensive re-crystallisation and re-equilibration of these ductile orthogneisses at ca. 400°C–450°C and 6 kbar. The consequential conclusion, that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these Dabieshan orthogneisses with undoubted UHP lithologies, has far-reaching implications for the interpretation of controversial gneiss–eclogite relationships in other UHP metamorphic terrains. 相似文献
13.
Hydrothermal fluids emerge from the seafloor of Paleohori Bay on Milos. The gases in these fluids contain mostly CO 2 but CH 4 concentrations up to 2% are present. The stable carbon isotopic composition of the CO 2 (near 0%) indicates an inorganic carbon source (dissociation of underlying marine carbonates). The carbon and hydrogen isotopes of most CH 4 samples are enriched in the heavy species (δ 13C = −9.4 to −17.8‰; δD = −102 to −189‰) which is believed to be characteristic for an abiogenic production of CH 4 by CO 2-reduction (Fischer-Tropsch reactions). Depletions in the deuterium content of three CH 4 samples (to −377%) are probably caused by unknown subsurface rock alteration processes. Secondary hydrogen isotope exchange processes between methane, hydrogen and water are most likely responsible for calculated unrealistic methane formation temperatures. We show that excess helium, slightly enriched in 3He, is present in the hydrothermal fluids emerging the seafloor of Paleohori Bay. When the isotopic ratio of the excess component is calculated a 3He/4Heexcess of 3.6 · 10−6 is obtained: This indicates that the excess component consists of about one third of mantle helium and two thirds of radiogenic helium. We infer that the mantle-derived component has been strongly diluted by radiogenic helium during the ascent of the fluids to the surface. 相似文献
14.
40Ar/ 39Ar dating of metamorphic biotite and alteration muscovite from the auriferous veins and host rocks at the Hill End goldfield, N.S.W., Australia, has distinguished four major geological events, including the timing of gold mineralization. The earliest hydrothermal event occurred during the Middle Devonian Tabberabberan Orogeny (370–380 Ma) and resulted in the formation of quartz veins barren of Au. A second and major episode of vein emplacement occurred in the Early Carboniferous during the principal phase of metamorphism and deformation at 359–363 Ma. This was followed by Au accumulation in two stages: (1) after the major phases of quartz deposition, and (2) during and after the development of conspicuous internal vein laminations (˜ 357 Ma and ˜ 343 Ma, respectively). Two sources of fluid are proposed for vein and ore formation. The first is a local metamorphic fluid characterized by δ 18O H2O values of 8.9 to 12.5 per mil and δD H2O values of −87 to −90 per mil. The second is a mixed ore fluid with δ 18O and δD values in the range of δ 18O H2O 8.4 to 11 per mil and δD H2O of −49 to −36 per mil. Progressive entry of this second fluid, sourced from trough-fill or deeper crustal rocks, is linked closely to cycles of gold precipitation at Hill End. 相似文献
15.
Several types of fluid immiscibility may affect the evolution of volatile-rich magmatic systems at the magmatic–hydrothermal transition. The topology of silicate–salt–H 2O systems implies that three-fluid immiscibility (silicate melt+hydrosaline melt+vapour) should be stable in a broad range of compositions and P– T conditions. The most important factor controlling the immiscibility appears to be the Coulombic properties (electric charges Z and ionic radii r) of the main network-modifying cations and the capacity for immiscibility appears to decrease in the following sequence: Mg>Ca>Sr>Ba>Li>Na>K. Liquid immiscibility is enhanced in peralkaline compositions and in the presence of nonsilicate anions such as F −, Cl −, CO 32− and BO 33−. In volatile-rich magmatic systems, the H 2O is likely to react with the chloride, fluoride, borate and carbonate species and the chemical effects of high-temperature hydrolysis may be greatly enhanced by phase separation in systems with multiple immiscible fluid phases. Natural granitic magmas can thus exsolve a range of chemically and physically diverse hydrosaline liquids and the role of these fluid phases is likely to be especially significant in pegmatites and Li–F rare-metal granites. 相似文献
16.
The abiotic synthesis of organic compounds in seafloor hydrothermal systems is one mechanism through which the subsurface environment could be supplied with reduced carbon. A flow-through, fixed-bed laboratory reactor vessel, the Catalytic Reactor Vessel (CRV) system, has been developed to investigate mineral–surface promoted organic synthesis at temperatures up to 400°C and pressures up to 30 MPa, conditions relevant to seafloor hydrothermal systems. Here we present evidence that metastable methanol can be directly synthesized from a gas-rich CO 2–H 2–H 2O mixture in the presence of a mineral substrate. Experiments have been performed without a substrate, with quartz, and with a mixture of quartz and magnetite. Temperatures and pressures in the experiments ranged from 200°C to 350°C and from 15 to 18 MPa, respectively. Maximum conversion of 5.8×10 −4% CO 2 to CH 3OH per hour was measured using a mixture of magnetite and quartz in the reactor. After passivation of the stainless steel reactor vessel, experiments demonstrate that methanol is formed at temperatures up to 350°C in the presence of magnetite, and that the formation rate decreases over time. The experiments also show a loss of surface reactivity at 310°C and a regeneration of surface reactivity with increased temperature up to 350°C. Concentrations of CO 2 and H 2 used in the experiments simulate periodic, localized and dynamic conditions occurring within the seafloor during and immediately following magmatic diking events. High concentrations of CO 2 and H 2 may be generated by dike injection accompanied by exsolution of CO 2 and reaction of dissolved H 2O with FeO in the magma to form H 2. The experiments described here examine how the ephemeral formation of an H 2–CO 2-rich vapor phase within seafloor hydrothermal systems may supply reactants for abiotic organic synthesis reactions. These experiments show that the presence of specific minerals can promote the abiotic synthesis of simple organic molecules from common inorganic reactants such H 2O, CO 2 and H 2 under geologically realistic conditions. 相似文献
17.
There are two types of gneisses, biotite paragneiss and granitic orthogneiss, to be closely associated with UHP eclogite at Shuanghe in the Dabie terrane. Both concentration and isotope composition of bulk carbon in apatite and host gneisses were determined by the EA-MS online technique. Structural carbonate within the apatite was detected by the XRD and FTIR techniques. Significant 13C-depletion was observed in the apatite with δ13C values of −28.6‰ to −22.3‰ and the carbon concentrations of 0.70–4.98 wt.% CO 2 despite a large variation in δ18O from −4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ13C and δ18O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in 13C primarily, but subjected to overprint of 13C-rich CO 2-bearing fluid after the UHP metamorphism. The 13C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric–hydrothermal alteration before plate subduction. Both low δ13C values and structural carbonate in the apatite suggest the presence of 13C-poor CO 2 in the UHP metamorphic fluid. The 13C-poor CO 2 is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism. Zircons from two samples of the granitic orthogneiss exhibit low δ18O values of −4.1‰ to −1.1‰, demonstrating that its protolith was significantly depleted in 18O prior to magma crystallization. U–Pb discordia datings for the 18O-depleted zircons yield Neoproterozoic ages of 724–768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie–Sulu orogen. Therefore, the meteoric–hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated with the Rodinia supercontinental breakup and the snowball Earth event. It is thus deduced that the igneous protolith of the granitic orthogneiss and some eclogites would intrude into the older sequences composing the sedimentary protoliths of the biotite paragneiss and some eclogites along the northern margin of the Yangtze plate at mid-Neoproterozoic, and drove local meteoric–hydrothermal circulation systems in which both 13C- and 18O-depleted fluid interacted with the protoliths of these UHP rocks now exposed in the Dabie terrane. 相似文献
18.
The Tarçouate pluton (Anti-Atlas, Morocco) is an inversely zoned laccolith emplaced 583 Ma ago into low-grade metasediments, with the following succession: leucocratic granites, biotite–granodiorites (±monzodiorites), hornblende–granodiorites (±monzodiorites) and monzodiorites syn-plutonic dykes. These rocks form two distinct, chemically coherent, units: (1) A main unit consists of layered (572<59 wt.%) and homogeneous (632<67%) hornblende–granodiorites, biotite–granodiorites (672<72%) and aplites (702<76%). All these rocks are metaluminous to peraluminous and display fractionated HREE depleted patterns (La/YbN=14–61; YbN=0.7–6.8). Initial 87Sr/86Sr ratios (0.7072 to 0.7080) increase, whereas Nd(t) values (−1.7 to −2.8) decrease from the hornblende– to the biotite–granodiorites. Monzodiorites occur as mafic microgranular enclaves or syn-plutonic dykes. (2) A subordinate unit consists of leucocratic, distinctly peraluminous, muscovite-bearing granites (722<75%) occurring at the northern edge of the pluton and as dykes in the surrounding schists towards the top of the pluton. These rocks are free of monzodioritic enclaves. They display less fractionated patterns with higher HREE contents (La/YbN=2–19; YbN=11–18), a distinct Nd(t) value (−11.8) and a 87Sr/86Sr initial ratio (0.7480) within those of the surrounding schists (0.7393–0.7819). Magma–host interactions are closely related to differentiation and occurred at different levels, but mainly before emplacement. Field relationships and petrogenetic modelling show that the bt–granodiorites formed at levels deeper than the level of emplacement, by fractional crystallisation (0.65These data preclude any significant material transfer process for the emplacement of the Tarçouate pluton, but rather suggest assembly of successive pulses of variably differentiated crystal-poor magmas. These shallow level granitic plutons can be considered as an end-member of magma emplacement with minimum interactions with the country rocks. 相似文献
19.
The Qinling–Dabie–Sulu belt is the world's largest ultrahigh pressure (UHP) metamorphic belt. The UHP metamorphism is well dated at 220–245 Ma in the Dabie–Sulu belt but at 507 Ma in the Qinling belt. The Tongbaishan is located between the Qinling orogenic belt to the west and the Dabie–Sulu UHP metamorphic belt to the east. It is the key area for studying the tectonic relation between the Qinling and Dabie–Sulu belts and the diachronous UHP metamorphism. The Jigongshan granitic pluton ( t=128 Ma) with a total area of 1200 km 2, composed of monzogranite, was mostly emplaced into the Tongbai complex, an exposed basement in the Tongbaishan. The Jigongshan granites have SiO 2=69.85–72.35%, K 2O/Na 2O=0.87–1.13, A/CNK=0.91–1.03, Rb/Sr=0.14–0.25 and Th/U=3.3–12. Their REE compositions show strongly fractionated patterns with (La/Yb) N=14–58 and Eu*/Eu=0.79–1.05. The granites are characterized by low radiogenic Pb isotopic composition. The present-day whole-rock Pb isotopic ratios are 206Pb/ 204Pb=16.707–17.055, 207Pb/ 204Pb=15.239–15.326 and 208Pb/ 204Pb=37.587–37.853, which are similar to that of the continental lower crust. Their Nd( t) values range from −16 to −20, and depleted-mantle Nd model ages ( TDM) from 1.8 to 2.2 Ga. The above evidence indicates that the magma of the Jigongshan granites was derived from the partial melting of the continental crust. The Pb and Nd isotopic compositions of the Jigongshan granites resemble those of the Dabie core complex in the Dabieshan but are distinct from those of the Tongbai complex in the Tongbaishan. Thus, the Dabie core complex would be the magma source of the Jigongshan granites. The result implies that the Dabie core complex is extended to the west and constitutes the unexposed basement underlaying the Tongbai complex in the Tongbaishan. 相似文献
20.
The Attepe district consists of Precambrian, Lower–Middle Cambrian, Upper Cambrian–Lower Ordovician and Mesozoic formations. It contains several iron deposits and occurrences. Three types of iron-mineralizations can be distinguished in the area; (i) Sedimentary Fe-sulfide in Precambrian bituminous metapelitic rocks, and Fe-oxides in Precambrian metasandstones (SISO), (ii) vein-type Fe-carbonate and oxides composed of mainly siderite, ankerite and hematite including barite in Lower–Middle Cambrian metacarbonates of the Çaltepe Formation (HICO), (iii) karstic Fe-oxides and hydroxides essentially in the Lower–Middle Cambrian metacarbonates and the unweathered Fe-carbonates (KIO). The latter type is more widespread and located at the upper parts of the most important mineable iron deposits like Attepe deposit. Oxygen-, carbon-, sulfur- and strontium-isotope studies have been performed on siderites and barites in the vein-type ores, and on calcites in the recrystallized Çaltepe Limestones to investigate the sources and formation mechanism of primary ore-forming constituents. The δ13C values of siderites and calcites in limestones of the Çaltepe Formation range from −10.10‰ to −8.20‰, and from −0.8‰ to 2.30‰. Both carbonate minerals show δ18O values between 17.50–18.30‰ and 16.20–23.00‰, respectively. The δ13C and δ18O isotopic variations do not indicate any direct or linear relations between siderites and limestones. However, it is possible that the carbon and oxygen isotopic compositions of carbonate minerals could be changed to some extent, when limestones were subjected to hydrothermal processes or thermal alterations during metamorphism. The isotopic values of barites display 32.40–38.30‰ for δ34S and 12.20–14.70‰ for δ18O. The strontium isotope ratios (0.717169–0.718601) of barites and the sulfur isotope compositions of barites and pyrites suggest that there are no direct linkages of ore-forming compounds neither with a magmatic source nor with sedimentary pyrite formations in the Precambrian bituminous shales of the Attepe formation. According to the field observations and the stable isotope data, siderites and ankerites should be formed by interaction between iron-rich hydrothermal fluids and Çaltepe limestones, whereas isotope ratios of barites indicate that they were formed by mixing of sulfur-rich meteoric waters and deeply circulated hydrothermal solutions. 相似文献
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